Generally, a reading device detects a size of an original document to be read, sets a region of the detected document size as a reading range, and reads the original document.
A reading device known in the art determines a document size of an original document using a scanner which moves in a moving direction and detects a document width of the original document at a home position provided with a predetermined interval from a leading edge of the original document in the moving direction and a sensor which detects a document length of the original document.